Current theories regarding encoding and processing of auditory information have as their starting point a large body of detailed information on response properties of single auditory nerve fibers in cats, yet our understanding of the underlying organization of the primary afferent inputs to the cochlear nucleus (CN) is remarkably limited. The overall objective of the proposed studies is to provide fundamental information about the morphological organization and connections of cochlear spiral ganglion neurons in adult cats. Physiological recording and cytochemical labeling techniques combined with extensive light and electron microscopic analyses will be employed in these studies. Among our objectives are the following: 1) To map in fine detail the topography of the spinal ganglion projection to ventral cochlear nucleus subdivisions and cell types, utilizing HRP microinjections into the spinal ganglion; and in these experiments, to further define how the frequency representational dimension and intraganglionic vertical position project in detail onto the striking "isofrequency laminae" of the major subdivisions of the CN. 2) To determine how spiral ganglion cells subserving different known classes of IHC afferent terminals are distributed in their projections across the vertical and longitudinal dimensions of Rosenthal's canal. 3) To define the topographic distributions of certain physiological response properties (among them, characteristic frequency, threshold, spontaneous discharge rate, latency, Q-10dB) within and between the "isofrequency laminae" of ventral cochlear nucleus divisions, and to relate those findings to the morphology of CN projection patterns demonstrated in HRP experiments. 4) To determine whether CN projection patterns remain static or undergo topographic reorganization after ototoxic drug induction of profound sensorineural deafness and consequent severe degeneration and alteration of spiral ganglion neurons. These proposed experiments will define in much richer detail the organization of the cochlear projections to the CN, and thereby increase our understanding of the anatomical framework of the information processing machinery of the peripheral auditory system. Such data are important for formulating more complete models of the central processing of auditory information and should be of critical interest to many investigators. Proposed studies in pathological cochleas are of clinical significance, as they further define: 1) factor(s) that induce degeneration of the cochlear nerve; and 2) the nature of the CN projections from ganglion cells surviving severe deafness pathology. Data from these unique studies documenting the time course and extent of alteration in CN inputs after deafness onset have practical implications regarding the efficacies and optimization of cochlear implants in patients with similar pathologies.